The field of the invention is slide projectors having systems for automatically focusing the slides to be projected.
The state of the art may be ascertained by reference to U.S. Pat. Nos. 3,399,595; 3,519,992 and 3,639,047, the disclosures of which are incorporated herein. FIG. 4 of U.S. Pat. No. 3,517,992 shows an embodiment of an automatic focusing slide projector wherein the slide guide means is kept stationary, except for a slide changing movement perpendicular to the optical axis, and the projection lens is displaceable along the optical axis as in the present invention.
The present invention relates particularly to a slide projector with an automatic focusing system consisting of a scanning auxiliary projecting system and a position control focusing the slide and comprising a light sensor for the reflections from the slide.
Systems of this kind are known as indicated above. The light detectors used conventionally are photoresistors which recently have included a proportion of selenium to improve the detection. This inclusion of selenium extends the sensitivity into the infrared region of the spectrum, which is advantageous and the insertion of the filters reduces the stray light effect which occurs at the slide when the main illumination beam passes through it and which is almost entirely within the visible spectral range. But, however, filtering also reduces the intensity of the infrared (IR) beams so that a more powerful light source is required for the auxiliary projecting system.